U.S. Pat. No. 3,886,372 to B. Sanglert and U.S. Pat. No. 3,983,403 to C. Dahlstrom et al exemplify the methods and apparatus to which the present invention generally relates. In such methods and apparatus, photoresponsive devices are employed to scan a workpiece and produce signals that can be fed to a computer to enable it to make an evaluation of geometrical features of the workpiece.
A typical workpiece to be scanned with such apparatus is a cant or similar unfinished lumber piece having at least one surface that has been sawed to flatness and having at least one wane surface which is oblique to the sawed surface and which is to be trimmed away in a finishing operation. The information obtained from scanning is employed for calculation of trimming cuts that will convert the particular workpiece into a finished piece of lumber having the highest obtainable economic value, taking into account both market conditions and the configuration of the unfinished workpiece itself. The optimizing calculations can be performed by known data processing equipment, programmed in a known manner, but the accuracy of the results obtained with such equipment is necessarily dependent upon the sufficiency and the accuracy of the data fed into it.
In the prior systems for the automatic measurement of cants and similar unfinished lumber workpieces, as in the system of this invention, an aligned array of photosensitive detector elements scans along elongated zones on the workpiece, which zones extend transversely to the direction in which the detector elements are aligned. Scanning is effected by relative movement between the detector array and the workpiece, produced in any suitable manner. Thus the Dahlstrom et al patent discloses that the array can be moved while the workpiece is held stationary, or that the workpiece can be moved past a stationary array; and the Sanglert patent discloses that a swinging or rotating mirror can be interposed in the light path between the workpiece and the array to cause successive parts of a stationary workpiece to be imaged onto a stationary array as the mirror swings. Moving the array is unsatisfactory for rapid scanning because of its bulk and the cable connections to it, and the use of a swinging mirror introduces mechanical complexity and increases the changes for measurement errors. On the other hand, prior systems in which the workpiece moved during scanning tended to be slow and inaccurate.
In whatever manner the workpiece has been scanned, it has heretofore been necessary to so illuminate it that one of its wane or side surfaces was put into relative shadow during one part of the scanning operation and its other wane was shadowed during another part of the scan. In accordance with the teachings of U.S. Pat. No. 3,890,509 to C. W. Maxey, this was done by illuminating the workpiece alternately from each of two light sources that cast light laterally across the top surface of the workpiece at opposite low oblique angles. Thus, with illumination of each light source, the wane or edge that was away from it would be in shadow, and there would be a relatively abrupt drop in magnitude of the output signal from each photoresponsive element as its scan moved onto the shadowed wane or edge.
With prior scanning systems wherein such illumination was employed, the workpiece was either scanned twice--once with illumination from each source--or the lighting was changed when the scan had progressed about halfway across the workpiece. With two scans the workpiece had to be stationary during scanning, and the processing rate tended to be slow, whereas single-scan systems tended to be less accurate.
With both types of prior systems it was possible to produce output signals from the photoresponsive array that defined the location of edges of a workpiece and the edges of its wane surfaces, but no other information was obtainable about the workpiece geometry and configuration. Thus, if a scanned top surface of the workpiece was substantially curved across its width, the scanning outputs did not signify information about such curvature.
In those prior scanning systems wherein the workpiece had to be stationary at the measuring station at which scanning took place, relatively complex mechanism was needed for alternately moving and stopping the workpieces; but, more important, stopping each workpiece at the measuring station markedly slowed the flow of workpieces along the processing line, and accelerations of workpieces as they were moved out of the measuring station could cause their orientations to shift so that the measurement data become inaccurate or meaningless.
In prior systems it was considered necessary that the light or invisible radiation used for scanning have a substantially constant level and be of relatively high intensity so that reflection from the sawed top surface of the workpiece would contrast strongly with that from a shadowed wane surface, to ensure a sufficient change in output signal level for accurate identification of the boundary edge between those surfaces. Since prior systems required a perceptible alternation in the operation of the two light sources, the use of constantly changing visible light at a high intensity level was fatiguing to persons who had to work in the vicinity. Of course high intensity illumination also has the disadvantage of high energy consumption.
In such systems as those of U.S. Pat. Nos. 3,983,403 and 3,963,938, measurement errors could arise because different detector elements of an array camera respectively scanned the wane surfaces and the sawed top surface of a cant. It often happens that individual elements in an array have response characteristics which are not accurately matched to the response characteristics of the other elements, with the result that such individual elements produce outputs that are inconsistent with those from the other elements. It can also happen that amplifier drift can affect the outputs of one or several detector elements. Such output signal inconsistencies and distortions will of course affect the accuracy of the measurements made with the scanning system unless compensation is made for them, and heretofore it has not been known how to effect suitable compensation.